The use of dispersions of coal in water or oil as a fuel in place of residual oil is well known. The objective in using such dispersions is to take advantage of the economy and availability of coal in a fluid fuel which can be easily transported and atomized for combustion. Of course, since the objective is to replace fuel oil, the dispersions of coal in water are of especial interest, provided that such dispersions can be loaded with sufficient coal to avoid a significant fuel value debit.
Dispersions of coal tend to sediment while quiescent. (That is, the particulate coal settles to the bottom of a storage tank, resulting in a non-homogeneous dispersion.) Such non-homogeneous dispersions are difficult to burn efficiently as the fuel value will vary with amount of particulate coal that is found in the dispersion. Dispersions of coal may therefore be agitated continuously or made up just prior to use to avoid sedimentation problems. Both approaches are not completely satisfactory, and in fact at times it is impossible to redisperse sedimented coal without heroic efforts.
Various methods have been utilized to stabilize dispersions of coal and to allow redispersion of the coal without expending large amounts of mechanical energy. For example, it is known that more finely ground coal is easier to disperse and also once such finely ground coal is dispersed, it exhibits a tendency to avoid sedimentation. Therefore, one approach to stabilizing dispersions of coal has been to grind the coal to a very fine average particle size.
Intensive grinding requires very large amounts of mechanical energy, and therefore to some extent defeats the economical advantage of using coal dispersions in place of residual or fuel oil. That is, the major reason that it is desirable to substitute dispersions of coal for fuel oil in boilers and other combustion means is that coal is more economical and more highly available energy source than fuel oil. Thus, a process requiring the input of energy to more finely grind the coal is somewhat counterproductive.
It is also known that increasing viscosity of the coal dispersions provides resistance to sedimentation. However, the higher viscosity dispersions are difficult to handle and burn. That is, it is well known that above a certain viscosity it is difficult to transport coal-dispersions between storage vessels and combustion facilities. Moreover, efficient burning requires complete atomization in the combustion zone of the boiler. High viscosity hinders such complete atomization. Therefore, although high viscosity coal dispersions are resistant to sedimentation, the problems caused by such high viscosity may outweigh the benefits.
Various investigators have utilized additives to overcome the above problems. For example, Krause et al, in U.S. Pat. No. 4,101,293 teach that dispersions of coal in fuel oil may be made stable to storage by incorporating a stabilizer prepared by reacting blends of unsaturated aliphatic and cycloaliphatic carboxylic acids with alkali metal hydroxides or amines. The dispersions taught in such patent are limited to oil continuous dispersions unlike the dispersions of the present invention and are loaded with no more than 50%, by weight, coal. Moreover, it is clear (as discussed below) that the incorporation of alkali metal or nitrogen moieties (from the alkali metal hydroxide and the amine, respectively) is undesirable from the standpoint of ash handling and pollution.
Meyer, in U.S. Pat. No. 4,130,400, avoids the use of an additive which causes pollution and ash handling problems, by means of an additive comprising a copolymer of alkylstyrene. Such additive would not be effective in stabilizing dispersions of coal in water since it is insoluble in water. In fact, the patentee notes that one of the objects of his invention is to avoid the incorporation of water in the disclosed dispersions.
Yamamura et al, in U.S. Pat. No. 4,330,301, discloses that a sulfonated polynuclear dispersant is useful in dispersing coal in water. Again, the utilization of sulfur moieties and the sodium, calcium, ammonium salt neutralizing agents, as taught by the patentees is undesirable from a pollution and ash handling standpoint.
Braun et al, in U.S. Pat. No. 4,242,098, teaches that the addition of small amounts of water-soluble polymers to aqueous coal slurries permit formulation of a mobile slurry containing up to 78% coal. The water-soluble polymers which can be used include poly(ethylene oxide), partially hydrolyzed poly(acrylamide), hydroxyethyl cellulose, quaternary nitrogen-substituted cellulose ethers, xanthan gum, hydroxypropyl guar gum, and carboxymethyl hydroxypropyl guar gum. The patentee is only concerned with transporting coal wherein the dispersion is continuously agitated and does not consider the use of multi-component additive systems to provide a dispersion of coal in water which is stable to storage, and easy to burn, as well as easy to transport.
Finally, Burgess, in U.S. Pat. No. 4,304, 573, indicates that the prior art worker will go to an extreme to prepare dispersions of coal. The indicated approach requires the graft polymerization of monomers on the coal surface to render such surface hydrophobic and oleophilic. No such extreme is contemplated in the process of the instant invention.